1. Field of the Invention
This invention relates to vinyl monomer compositions exhibiting accelerated surface cure characteristics. These compositions are prepared from a vinyl monomer or monomer mixture, such as a blend of dicyclopentenyloxyethyl methacrylate and hydroxypropyl methacrylate, a bulk phase polymerization catalyst system, a polyvalent metal oxidative surface phase catalyst, and a diolefin rubber dispersed in the vinyl monomer as a surface cure accelerator. These compositions are useful as binders for polymer concrete and as impregnants for porous surfaces. The accelerated surface cure is particularly advantageous with monomer mixtures containing low volatility vinyl monomers.
2. Brief Description of the Prior Art
Vinyl monomer compositions which are polymerized in situ to form protective coatings are well known. For example, methyl methacrylate compositions have been widely studied as impregnants or sealers for concrete, and as binders for aggregate-filled "polymer concrete" composition useful for coating or patching portland cement based concrete. Typically, the substrate is pretreated to enhance penetration of monomer and the impregnated or coated substrate is cured in a subsequent polymerization step. Concrete treated in this manner can be more resistant to abrasion and chemical spills than untreated concrete. Vinyl monomer may be polymerized in bulk by free radical polymerization initiated by high energy radiation, particle beams or chemical sources of free radicals such as peroxides and hydroperoxides. On the other hand, it is well known that free radical polymerization of vinyl monomers may be inhibited by molecular oxygen. The effect of oxygen inhibition on polymerization becomes particularly troublesome in surface coating and impregnant compositions, such as those used in protecting concrete surfaces. In the special case of compositions based on methyl methacrylate, the lack of surface cure is not particularly disadvantageous. Methyl methacrylate has significant vapor pressure and uncured monomer on the surface of an otherwise polymerized composition volatilizes relatively quickly, exposing the hard polymeric surface beneath.
However, when the uncured composition contains less volatile vinyl monomer, the surface of composition which has been cured in bulk may be very slow to cure because of oxygen inhibition. The results in a surface with such undesirable properties as tack and persistent residual odor. Compositions containing low volatility monomers may be otherwise desirable because of enhanced chemical resistance, greater safety, low odor and other properties which such monomers confer when polymerized. Under-cured oligomeric material on the surface may increase the water sensitivity.
A variety of techniques have been used in an attempt to solve the problem presented by inhibition of polymerization by oxygen. For example, a polyvalent metal salt such as zinc naphthenate may be included in the coating or impregnant composition in order to catalyze the oxidative cure of the residual uncured monomer. Driers such as zinc naphthenate are commonly used to promote through-drying of alkyd paints. In addition, in a manufacturing context, articles coated with vinyl monomer composition can be cured under a blanket of nitrogen or other inert gas. Alternatively, a second coating of a material such as paraffin, which reduces oxygen penetration at the surface of the applied vinyl monomer, can also be used to ameliorate the situation. The paraffin can be included with a blend of volatile monomers to yield a surface film on application. Alternatively, a second film of paraffin can be applied over the initial coating film. None of these solutions to the problems arising from oxygen inhibition of surface cure has been totally satisfactory, and there remains a significant need for vinyl monomer compositions which rapidly develop surface cure, especially in the case of compositions which contain low volatility vinyl monomers.